1. Field of the Invention
The present invention relates to an improvement of cooling of a steel strip which has been cooled through a cooling zone in a continuous heat treating line, in particular, of final cooling of the strip by immersing in cooling water in a cooling tank.
2. Related Art Statement
There has been heretofore employed such method of cooling the steel strip by continuously passing through cooling water in a cooling tank for finally cooling the strip in the continuous heat treating line such as a continuous annealing line.
The cooling tank used for cooling the steel strip is provided with a sensor for detecting temperature of cooling water, a pump for supplying cooling water and a temperature controller and arranged such that the strip is cooled to a predetermined temperature during immersing in the cooling water in the cooling tank, while the cooling water is heated by taking the heat energy of the strip so as to be recovered in the form of hot water. Such steel strip cooling method is described, for example, in Japanese Patent Application Publication No. 11,933/57.
There has been however known that when the steel strip having a high temperature is cooled by immersing in cooling water in the cooling tank, the surface of the steel strip is often dirtied with foreign substances such as dirty suspensions or the like in the cooling water.
Furthermore, it has been known that the tendency of dirt adhesion on the surface of the steel strip becomes higher as in particular the temperature of the steel strip at the inlet of the cooling tank is higher and the amount of steel strip to be cooled in the cooling tank is greater.
It has been found that the surface of the steel strip is dirtied as a result in that in case of the steel strip still having a high temperature at the inlet of the cooling tank after cooling through the cooling zone in the heat treating line, the strip can not be sufficiently cooled with the cooling water in the cooling tank by the time of contacting with a first sink-roll so that a water film interposed between the surface of the sink-roll and the surface of the strip which is wound around the sink-roll is evaporated by the heat of the strip having a high temperature to deposit dirty suspensions included in the water on the surface of the strip.
Accordingly, in order to reduce the temperature of the steel strip at the time of winding the strip around the sink-roll, some methods have been proposed such that the steel strip is sufficiently cooled through the cooling zone of the heat treating line to fall the temperature of the strip at the inlet of the cooling tank or the cooling tank is made larger to increased the distance from the surface of cooling water to the sink-roll so as to cool the strip sufficiently with cooling water until the strip reaches the first sink-roll.
Such methods however have disadvantages that in case of reducing the temperature of the steel strip at the inlet of the cooling tank, not only the heat energy of the strip can not be recovered by the cooling water, but also the electric opower consumed in cooling the strip through the cooling zone arranged before the cooling tank is increased and in case of using the larger cooling tank, the cost of equipment becomes higher.
An object of the present invention is to provide a method and an apparatus of finally cooling a steel strip capable of preventing dirts from adhering to the surface of the strip without the above mentioned disadvantages.
Another object of the invention is to provide a method and an apparatus of cooling a steel strip capable of using a smaller cooling tank.
A further object of the present invention is to provide a method and an apparatus of effectively cooling a steel strip having a higher temperature at the inlet of the cooling tank to substantially reduce the power consumed in cooling the steel strip in the cooling zone of the continuous heat treating line.
According to an aspect of the present invention, a method of cooling a steel strip which has been cooled through a cooling zone in a continuous heat treating line comprises steps of immersing the strip in cooling water through around one or more sink-rolls in a cooling tank and injecting cooling water jets to the strip from injection nozzles arranged in the cooling water until the immersed strip reach the first one of the sink-rolls, thereby to cool the strip to a temperature for preventing evaporation of a water film interposed between the surface of the first sink-roll and the surface of the strip wound around the first sink-roll.
In a preferable embodiment of the invention, the injection of water jets from the injection nozzles may be controlled in accordance with the following formula: ##EQU1## here, l is the length of the portion of a steel strip cooled by water jets injected from injection nozzles (m)
Ts is the temperature of the steel strip at the inlet of the cooling tank (.degree.C.) PA1 Tw is the temperature of cooling water (.degree.C.) PA1 Cp is the specific heat of the steel strip (Kcal/kg.degree.C.) PA1 v is the feed speed of the steel strip (m/hr) PA1 d is the thickness of the steel strip (m) PA1 .alpha. is the coefficient of heat transfer (8,500.about.10,500 Kcal/m.sup.2 hr.degree.C.) PA1 .rho. is the density of the steel strip (kg/m.sup.3)
According to another aspect of the present invention, an apparatus for cooling a steel strip which has been cooled through a cooling zone in a continuous heat treating line comprises a cooling tank containing cooling water, one or more sink-rolls arranged in the cooling water to guide the steel strip in the cooling tank, a guide roll provided at the inlet of the cooling tank for guiding the steel strip from the outlet of the cooling zone to the first one of the sink-rolls in the cooling water, a plurality of injection nozzles arranged along a passage of the steel strip in the cooling water to inject cooling water jets against the surfaces of the steel strip over the distance from the surface of the cooling water to the first sink-roll and means for supplying cooling water to the injection nozzles.
In a preferable embodiment of the invention, the apparatus further comprises a controller for controlling the temperature of the cooling water (Tw) and/or the steel strip (Ts) at the inlet of the cooling tank in accordance with the following formula: